I am developing an android app, it is a social media app with many features. One of the features is a private messaging between users. What is the best way to do it. Note: I'm using PHP for the backend of the app.
There are many ways you can Achieve it
You should take a look at Firebase Cloud Messaging (FCM)
It would help you create a Real time Chat Feature between a Single User and Multiple User
Also with Firebase Storage Capability you can also provide feature like Uploading Pictures and Videos